Electrolytic production of elemental boron



Unite States Patent 3,030,284 ELECTROLYTIC PRODUCTION OF ELEh/[ENTAL BORON David R. Stern, Fullerton, Calif.,.assignor to American Potash & Chemical Corporation, a corporation of Delaware No Drawing. Filed Nov. 3, 1960, Ser. No. 66,902 4 Claims. (Cl. 204-60) This is a continuation-in-part of application Serial No. 800,089, filed March 18, 1959, in turn a continuation-inpart of application Serial No. 574,605, filed March 29, 1956, both now abandoned.

This invention relates to the production of highly purified elemental boron by electrolysis, and more particularly to a process of the foregoing type which may be carried out more or less continuously for extended time periods.

Boron can be produced chemically by the reduction of boron oxide (B 0 with magnesium and other reducing metals. However, the efliciency on the utilization of the reducing metal, the formation of borides, the high temperature of reaction, and the difliculty of controlling the reaction conditions has resulted in a process giving low yields and a costly product, which always contains appreciable quantities of suboxides.

Production of boron by electrolysis of various borates and other compounds has been proposed heretofore by various investigators. Some of these have involved the utilization of relatively expensive boron-containing materials, while others required temperatures on the order of 1l00 to 1200" C. In any case, the resulting boron secured from the borates usually contained insoluble contaminates which lowered the quality of the product. However, instead of pointing the way to successful production of low-cost boron, these prior art processes have been misleading in that the investigators have stated that the occurrence of suboxides in the product has represented a barrier to the preparation of a high purity elemental boron.

It is therefore an object of this invention to provide for the production of a purified elemental boron by a process which enables high yields and which may be carried out commercially on a more or less continuous basis.

Other objects and advantages of this invention, if not specifically set forth, will become apparent during the course of the description which follows.

It has now been found that elemental boron of a relatively high purity may be formed by the direct electrolytic conversion of B 0 to elemental boron by carbon reduction,the chemistry, in short or summary form, being represented by the following:

Generally, it has been found that the process for producing boron comprising electrolyzing a fused bath by passing a current between a consumable carbon anode and an inert cathode may be carried out in a continuous fashion where the bath consists essentially of a mixture of boric oxide, an alkali metal oxide of the formula M 0, where M is an alkali metal, the boric oxide being present in a Weight concentration of at least 3% in excess of the percentage of boric oxide in the borate of the alkali metal corresponding to the alkali metal oxide having said formula M 0 and up to a total B 0 content in the bath of about 95%, the mole ratio of B 0 to M 0 being greater than two to one by such excess of B 0 It has also been found that the process may be carried out with unusual efliciency where a small amount of an alkali metal fluoride viscosity depressant is present in the bath.

More particularly, the process is carried out by electrolyzing a fused bath containing boric oxide as its major component and a small amount of an alkali metal oxide,

oxide.

e.g., sodium, potassium or lithium oxide. The proportion of boric oxide to alkali metal oxide should be such that the boric oxide is present in a concentration from at least 3% in excess of that required for an alkali metal borate of the formula M O-2B O where M is an alkali metal, up to a total B 0 content of about 95%. Thus, the minimum boric oxide content for a given alkali metal borate will be about 3% greater than the value given in the table below, the excess B 0 being derived by the addition thereof to the alkali metal borate. For example, with sodium borate, the B 0 content of the bath is from about 73% to about 95% on the total weight of alkali metal oxide-boric oxide mixture. The upper limit with regard to boric oxide is set by the high temperature required to electrolyze the mixture and the difficulty in providing a suitable container for the mixture, which is extremely corrosive at such high temperatures. This upper limit is usually from about to about boric Operation at the higher concentrations is preferred since the higher the boric oxide content, the greater is the purity of the product. The following table gives the boric oxide content of each suitable alkali metal borate.

TABLE I Percent B 0 content The chemistry of the process appears to be as follows:

2B O +3C 4B-I-3CO net result As noted above, it is desired to provide a certain amount of an alkali metal fluoride in the bath which serves as a viscosity depressant and increases current efiiciency. At no time during the course of the process should the alkali metal fluoride represent in excess of 10 weight percent of the total contents of the reaction vessel.

As may be seen from the above, the alkali metal fluoride does not enter into the reaction. It is also possible and it is within the scope of the claims hereinafter to substitute for a portion or all of the alkali metal fluoride a small amount of an alkali metal fluoborate, e.g., a quantity such that the alkali metal fluoride which forms therefrom never exceeds 10% of the total weight of the bath; an excess of MF, where M is an alkali metal, over the 10% level will cause the reaction to terminate.

The process of the invention may be carried out continuously for long periods of time, the only shut-down required being when the cathode and/or anode need replacement. As will be noted in the examples set forth below, this permits continuous operations without necessity for discarding the Na O-containing bath. If it necessary only that additional B 0 be added to maintain the level thereof above the minimum suggested earlier.

To practice the invention, a fused mixture of the desired proportions is charged into a crucible constructed of a suitable material, e.g., metal or graphite, which is protected by an external metal shell. The fused salts may be initially brought to operating temperature by external heating, induction heating, or resistance heating of the salts. An operating temperature of from 800 to l000 C. is preferred, the lower limit, of course, being the melting point of the mixture.

and current density between steel cathodes and graphite anodes. The passage of current is sufiicient to maintain the electrolyte at the desired temperature. At the be ginning of electrolysis, the voltage demand is low and steadily rises for a constant current input due to increased resistance resulting from the deposit obtained on the A layer of protective ferro-boron can be applied separately or in situ to the steel cathode to increase its useful life. The cathode can be removed and replaced occasionally and thus a substantially continuous electrolytic process is possible. particular advantage of this process is that the deposit on the c'athede is easily removed; when the cathode is placed in water it completely disintegrates Without any necessity for grinding. Since any electrolyte adhering to the deposit is water-soluble, no leaching problem is presented.

In the recovery of boron, I have not found evidence of any appreciable quantity of any boron suboxides; in the system sodium oXide-boric oxide, the major impurity in the product is sodium. When potassium oxide is used in place of sodium, there is a negligible metallic contamination. When boron of a particularly high purity is desired, the electrolyzed material containing sodium is readily treated to yield a high purity product. The purification is effected by heating the boron to an elevated temperature of about 1000 C. or higher to volatilize the sodium, preferably under reduced pressure, or at atmospheric pressure under an inert gas sweep. The recovered sodium is in elemental form. The apparatus employed for the boron purificationcan consist of a tantalum crucible inserted in a suitable housing to maintain an inert atmosphereand heated by induction heating.

Examples are set forth below for illustrative purposes, but they are not to be interpreted as imposing limitations on the scope of the invention other than as set forth in the appended claims.

EXAMPLES 1 AND 2 A cell body consisting of a cylindrically shaped cell, 24 inches in diameter by 26 inches high, made from mild steel sheet, was fitted with five graphite support pedestals which were placed within the cell and the cell filled with a layer of insulating powder up to the level of the support. A graphite crucible 20 inches CD. by 17 inches I.D., 18 inches high and 15 inches deep, was set on the supports and centered within the cell body by a permanent seated guide pin. Additional insulating powder was tamped between the graphite crucible and shell wall to the level of the crucible top. Several layers of insulating bricks were then placed over the packing. The cell head was next bolted onto the body. The head consisted of a steel plate having eight 1% inch diameter electrode ports. Four anode electrode ports were spaced equidistant on a inch circle and four heating electrode ports equally spaced on a 14 inch circle. The original head was provided with a cathode removal chamber, 4 inches in'diameter by 8 inches high of steel plate and welded to the cell head. The cooling chamber was provided with a /s-inch standard pipe nipple which served as an inert gas inlet, a flanged opening, and a matching chamber cover. During the second phase of the investigation a second cell cover was used without the cathode cooling chamber and in which both the anode and A.C. electrode ports were on the same hole circle (10 inch).

Neoprene gaskets were used between the cell head and shell and also at the closure of the cathode cooling top. The electrode ports were furnished with transite bushings which served as electrical insulators. The electrode connectors were fabricated of 2-inch copper rod stock with a %-10 N.C. copper stud on' the electrode end and /2-13 N.C. stud at the wire end. The connectors were wrapped with A-inch copper tubing for water cooling. Quick-disconnect fittings were placed on the tube lines which in turn were connected to hard rubber hoses. This arrangement enabled a change of electrodes when required with a minimum ofdown time.

4 Several grades of graphite were used as crucible material. These included National Carbon Company grade CS graphite and also grade code 82 graphite. Insulating bricks fabricated from National Carbon Company grade 20 porous carbon were used in several of the earlier runs. The bricks oxidized quite readily; thereafter, magnesia insulating bricks grade K-20 were used and proved quite adequate. Insulation powders used in the cell consisted of Ajax-Electrothermic Corporation Norblack, carbon black, and also Permanente Periclase, a granular form of silinienite refractory.

Auxiliary equipment included a panel board with voltmeter and ammeters for both the A.C. and DC. circuits", a Hoskins p yrometer with a chromel-alumel thermocouple for temperature measurements, and a Fischer and Porter purge meter to control the flow of argon gas.

DC. power was supplied to the cell by a 70-420 arnpere arc-welder. The A.C. heating electrodes were ener giied with power from a movable core transformer welder with a 420 ampere welding capacity. The transformer was limited to 70 amperes on the primary circuit and therefore limited the percent duty loading to abo ut 300 amperes. A second A.C. welder transformer was installed into the heating circuit during the latter stages of the study as a booster power supply. The booster transformer had a capacity of 7 kva.

Thecathode consisted of a13-inch piece of mild steel rod, 1 A-inch in diameter with a -inch hole drilled to 1 inch of the bottom, to accompany a thermocouple. The cathode rod was threaded of welded to a 27-inch length of thick-walled 7 Shelby tubing, %-inch diameter by -inch inside diameter. The Shelby tube shank was threaded with %-10 N.C. at the top and connected'to a copper bar. The negative D.C. lead, number 14 copper wire, was bolted to this bar. The cathode was immersed into the melt anywhere from 4-10 inches, depending upon the current densitydesired. For succeeding runs the cathodes were cleansed, buffed and reused. Wherever operating temperatures exceeded 950 C., a bright-silvery coating of ferro-boron formed on the rod. The coating did not appear to change the electrical characteristics of the cathode; therefore, no attempt was made to remove it from the rod prior to succeeding electrolyses.

Several grades of carbon were used as anode electrodes. These included the following:

National. Carbon Company-Grade AGX graphite National Carbon Company-Grade AGR graphite National Carbon Company-Grade GA carbon The usual anode electrodes measured l A-inch diameter by 24 inches long and were drilled and tapped on one end with A-IO N.C. female thread to accompany the water-cooled copper electrode connector.

The results of two different runs are presented in Tables II and III. Table II gives operating conditions while Table III gives the process raw material consumption on the basis of poundsof material required per pound of boron product.

As may be seen in Table II, this process may be carried out for an extremely long period of time and during that time substantial quantities of B 0 are consumed with very little, if any, Na O being consumed. In the first run, as can be seen in Table II and at the middle of Table III which summarizes the information obtained regarding pounds of B 0 and Na- O and carbon consumed per pound of product obtained, it will be seen that 0.82 pound of Na O per pound of boron product or a total of 6.03 pounds of Na 0 were consumed. This Na O consumption was due to drag-out, or the physical removalof Na O fromthe bath at the time that it was necessary to replace the carbon electrodes and to remove the boron product. In this run, work was done with a relatively viscous system with a result that the Na O clung to the anodes and cathodes and a certain quantity thereof was removed when the electrodes were taken from the bath.' Since the quantity of Na O present at the outset,

addition of either alkali metal fluoride or alkali metal fluoborate. In this event, the melt is somewhat more viscous and the current efliciency reduced somewhat. In

each of the examples which follow, the process was carhowever, Was so small, as contrasted with the quantity of 5 ried out briefly without the addition of a viscosity de- B O present, it will be understood that the removal of pressant. In each instance, an electrolyte of the indicated a certain quantity of Na O in this fashion is to be eX- composition was heated to the indicated temperature and pected where a viscous system is used. subjected to electrolysis under the voltage and current TABLE II Material Balance Data Product Raw Materials (Lbs.) Days Examples 1 and 2Electro- Total Conlytic System Amperetinuous Aver. Fed Consumed Hours Opera- Wt. Pertion (Lbs) centB B10: NazO NaF B103 N810 NaF Carbon 90 wt. percent Blot-10 wt. 44,947 58 7. 87 195.04 24.45 26.24 5.03 13.16

percent N820 90 Wt. percent BqOa5 wt. percent NatO-5 wt. percent NaF 212,663 78 26.1 88 534. 81 24.42 74.77 131.79 50.14 30.8

TABLE III Process Raw Materials Consumption Lbs/Lb. of Product D.O.Energy System Kw.Hours NaF Per Pound B10; N 9.10 Carbon Theore n 2. so 0 0. 72 3.96 0 90 Wt. percent 13203-10 Wt. percent NagO 3. 57 0.82 1. 79 251. 0 0 90 Wt. percent Bros-5 Wt. percent NazO-B Wt.

percent NaF 3. 05 0 1. 18 165. 0 1. 15

Where a small amount of a fluoborate is present at the outset, such fluoborate reacts with the alkali metal liberated at the cathode whereby to cause the reduction of the alkali metal according to the formula Where only a small amount of the alkali metal fluoborate is present at the outset, its presence may be disregarded once the reaction has proceeded even for a short period of time, since all of it is reduced and no longer afiects the reaction. However, the addition thereto of further alkali metal fluoborates so as to maintain its relative level in the melt would result in the continued liberation of the fluoride ion which in turn unites with sodium or potassium to form an alkali metal fluoride which cannot be electrolyzed and which builds up so as to require cessation of the process after only a limited period of time.

This procedure was carried out in the following example, wherein sodium fluoborate was used instead of the sodium fluoride, the sodium fluoborate being converted to sodium fluoride immediately after current flow commenced. The alkali metal fluoborate should preferably be consistent with the alkali metal oxide employed, e.g., sodium fluoborate should be used with sodium oxide, and potassium fluoborate should be used with potassium oxide.

It is also possible to carry out the process without the set forth. The product recovered on the cathode had the indicated composition. It will be observed that each of the operations was conducted at a temperature close to and preferably above the boiling point of metallic sodium, 880 C., for higher purity product. For this reason, some release of sodium from the cathode deposit may occur in the electrolytic cell. A massive liberation of sodium is not evident in the cell, however, and any vaporization of sodium occurs smoothly, very little being trapped within the cathode deposit so that there is a minimuum of sodium contamination of the boron.

EXAMPLE [Electrolyte B Os-10% K201 Product:

Percent boro 91.5 Percent HNO insol 1.1 Percent F 2.1 Percent K 0.09 Percent 0 5.2

Electrolysis data:

Current efficiency 45.2%. Volts 31.7. Current density 7.02 amps/sq. in. Temperature 1015 C.

EXAMPLE 5 [Electrolyte 90% B Oa- 10%LiO Product:

Percent boron 87.3 Percent HNO insol 2.9 Percent Fe 3.4 Percent Ti 1.3 Percent 0 5.1

Electrolysis data:

Obviously, many and various modifications may be made without departing from the spirit and scope of this invention, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim: I

1. A continuous process for producing boron comprising electrolyzing a fused bath by passing a current between a consumable carbon anode and an inert cathode, said bath consisting essentially of a mixture of an alkali metal fluoride representing no more than 10% by weight of the total weight of the said bath, boric oxide and an alkali metal oxide of the formula M 0, where M is an alkali metal, the boric oxide being present in a weight concentration at least 3% inexcess of the percentage of boric oxide in the borate of the alkali metal corresponding to the alkali metal oxide having said formula M Q and up to a total B O content in the bath of about 95%, the mole ratio of B to MO being greater than 2 to 1 by such excess of B 0 said alkali metal borate being of the formula M2B4O7.

2. The process of claim 1 wherein the alkali metal oxide is Na O and wheerin the alkali metal fluoride is NaF.

3. A continuous process for producing boron comprising electrolyzing a fused bath by passing a current between a consumable carbon anode and an inert cathode, said bath, consisting essentially of a mixture of boric oxide and an alkali metal oxide of the formula M 0, where M is an alkali metal, the boric oxide being present in a weight concentration at least 3% in excess of the percentage of boric oxide in the borate of the alkali metal corresponding to the alkali metal oxide having said formula M 0 and up to a total B 0 con-tent in the bath of about the mole ratio of B 0 to M 0 being greater than 2 to 1 by such excess of B 0 said alkali metal borate being of the formula M2B407.

4. A process as in claim 3 wherein the alkali metal oxide is sodium oxide and elemental boron is deposited on the cathode and contains sodium as an impurity, and purifying the boron by heating to an elevated temperature to volatilize the sodium.

References Cited in the file of this patent UNITED STATES PATENTS 2,848,396 Murphy et a1. Aug. 19, 1958 FOREIGN PATENTS 162,655 Great Britain Apr. 22, 1920 

1. A CONTINOUS PROCESS FOR PRODUCING BORON COMPRISING ELECTROLYZING A FUSED BATH BY PAASSING A CURRENT BETWEEN A CONSUMABLE CARBON ANODE AND AN INERT CATHODE, SAID BATH CONSISTING ESSENTIALLY OF A MIXTURE OF AN ALKALI METAL FLUORIDE REPRESENTING NO MORE THAN 10% BY WEIGHT OF THE TOTAL WEIGHT OF THE SAID BATH, BORIC OXIDES AND AN ALKALI METAL OXIDE OF THE FORMULA M2O, WHERE M IS AN ALKALI METAL, THE BORIC OXIDE BEING PRESENT IN A WEIGHT CONCENTRATION AT LEAST 3% IN EXCESS OF THE PERCENTAGE OF BORIC OXIDE IN THE BORATE OF THE ALKALI METAL CORRESPONDING TO THE ALKALI METAL OXIDE HAVING SAID FORMULA M2O AND UP TO A TOTAL B2O3 CONTENT IN THE BATH OF ABOUT 95%, THE MOLE RATIO OF B2O3 TO M2O BEING OF ABOUT 2 TO 1 BY SUCH EXCESS OF B2O3 SAID ALKALI METAL BORATE BEING OF THE FORMULA M2B4O7. 